THE THORAX AND RESPIRATION. 235 



bow, that string is stationary, while the bow, re- 

 presenting the rib, revolves ; thus the rib, by having 

 its anterior extremity depressed and revolving as 

 it is raised, enlarges the transverse diameter of the 

 thorax as well as the anterior diameter. In this 

 action the cartilage in front is twisted ; and the 

 torsion of this elastic matter affects the muscular 

 action in the manner following. 



We have understood the act of respiration to be 

 essential to life, and that the expansion of the chest 

 dilates the lungs, gives freedom of circulation 

 through them, and decarbonizes the blood. It is 

 interesting, therefore, to see how a property of 

 dead matter, elasticity, becomes a guard upon life. 

 Every one must feel that it is easier to expire the 

 air than to inspire it ; and if we can imagine a 

 person fainting, or in any mode in danger of death, 

 (the very word expiring, in its common sense, 

 implies that the last act of Hfe is the expulsion of 

 the breath,) if the elasticity tends to enlarge the 

 chest, it must tend to the preservation of life, by 

 restoring the circulation through the lungs. This 

 is exactly what happens from the elastic structure, 

 of the whole compages of the chest. The elastic 

 property preserves the chest in a middle state. 

 The muscles of inspiration act against the elastici- 

 ty : the muscles of expiration also act against it : 

 the elasticity tends, therefore, to maintain an in- 

 termediate state of dilatation of the thorax ; and 

 accordingly the lungs are preserved in a condition 

 to perform their functions for a certain period at 



